1. Field of the Invention
The present invention relates to a developing device for use in an electrophotographic image forming apparatus, to an image forming apparatus and to a method for filling developer. More particularly, the present invention relates to a trickle developing device that gradually supplies fresh developer and gradually discharge deteriorated developer, to an image forming apparatus incorporating the developing device and to a method for filling developer for use in the developing device.
2. Description of the Related Art
As developing systems employed for electrophotographic image forming apparatuses, the one-component developing system in which toner is used as the main component of the developer and the two-component developing system in which toner and carrier are used as the main components of the developer are known.
The two-component developing system that uses toner and carrier, in which the toner and carrier are charged by friction contact therebetween to predetermined polarities, has a characteristic that the stress on the toner is less than that in the one-component developing system that uses a one-component developer. Since the surface area of the carrier is larger than that of the toner, the carrier is less contaminated with the toner attached to the surface thereof. However, with the use for a long period, contamination (spent) attached to the surface of the carrier increases, whereby the capability of charging the toner is reduced gradually. As a result, problems of photographic fog and toner scattering occur. Although it is conceivable that the amount of the carrier stored in a two-component developing device is increased to extend the life of the developing device, this is undesirable because the developing device becomes larger in size.
To solve the problems encountered in the two-component developer, Patent document 1 discloses a trickle developing device being characterized in that fresh developer is gradually replenished into the developing device and developer deteriorated in charging capability is gradually discharged from the developing device, whereby the increase of the deteriorated carrier is suppressed. The developing device is configured to maintain the volume level of the developer inside the developing device approximately constant by discharging an excessive amount of deteriorated developer using the change in the volume of the developer. In the trickle developing device, the deteriorated carrier inside the developing device is gradually replaced with fresh carrier, and the charging performance of the carrier inside the developing device can be maintained approximately constant.
However, in a trickle developing device that uses a two-component developer, fresh developer is replenished from a replenishing tank and the developer existing inside the developing device is discharged from a discharge port to maintain the state in which the developer contained inside the developing device exists in a predetermined amount. Since the developer discharged from the discharge port contains unused carrier together with deteriorated carrier, there occurs a problem that the fresh carrier having been replenished intentionally is wasted. Accordingly, an image forming apparatus as disclosed in Patent document 2 is proposed to prevent replenished fresh developer from being discharged without being used.
In other words, the image forming apparatus according to Patent document 2 is characterized in that developer discharge operation is not carried out but circulation operation is carried out until the developer inside a first stirring and conveying chamber reaches a predetermined amount, and that when the developer having reached the predetermined amount is detected using a sensor, a lid having closed the discharge port of the first stirring and conveying chamber is opened and a shutter having closed the discharge port of the supplying and conveying chamber is opened, and newly replenished developer is circulated and stirred, whereby the newly replenished developer is prevented from being discharged directly.    [Patent document 1] Japanese Patent Application Laid-Open Publication No. Sho 59-100471    [Patent document 2] Japanese Patent Application Laid-Open Publication No. 2007-078757
In the image forming apparatus disclosed in Patent document 2, replenished fresh developer is discharged after circulated and stirred; since the shutter for discharging developer is configured so as to be driven using a shutter opening/closing motor on the basis of signals from a controller, there occurs a problem that the discharge control configuration for controlling the discharge of the developer is complicated and high in cost.
Accordingly, the technical problem to be solved by the present invention is to provide a developing device, an image forming apparatus and a method for filling developer capable of preventing carrier from being discharged without being used by using a simple configuration in a trickle developing device that uses a two-component developer.